Wave guide attenuator



0ct.20, 1953 w, E, GOOD 2,656,518

WAVE GUIDE ATTENUATOR Filed Jan. 15, 1946 INVENTOR MY/[amfl 600d.

, ATTORNE Patented Oct. 20, 1953 WAVE GUIDE ATTENUATOR William E. Good, Forest Hills, Pa., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application January 15, 1946, Serial No. 641,309

This invention relates to an attenuator for use in a high frequency system wherein electromagnetic waves are conducted through hollow wave guides.

It is often desirable in hollow wave guide systems to provide some means for attenuating the electromagnetic waves conducted through the guide. In the past, some difficulty had been encountered in providing suitable attenuation.

Much of this difficulty stems from the demand for an attenuator in which the amount of attenuation is readily adjustable while the refiection of power is maintained at a minimum so that the effect of the attenuator on the standing wave ratio is small.

It is an object of my invention to provide a new and improved attenuator for use in a hollow wave guide system.

Another object of my invention is to provide a novel attenuator for use in a hollow wave guide system with which the amount of attenuation may be readily adjusted.

A further object of my invention is to provide a new and improved attenuator for use in a hollow wave guide system which has little effect upon the standing wave ratio.

.A further object of my invention is to provide a new and improved attenuator for use in a hollow Wave guide system which has little effect upon the standing wave ratio and permits the amount of attenuation to be adjusted over a wide range.

In accordance with my invention, I provide a section of hollow wave guide with an elongated strip positioned within the guide and extending lengthwise thereof with the sides of the strip substantially parallel to the electric field of the electromagnetic waves to be conducted through the guide. The strip has a conductive surface layer with a resistance substantially greater than that of the inner surface of the guide. One point on the strip is secured to a wall of the guide with the strip extending away from that point in angular relation to the wall, the distance between another point on the strip and the Wall to which the strip is secured being readily adjustable.

The features of my invention which I consider novel are set forth with greater particularity in the accompanying claims. The invention itself, however, together with the advantages and additional objects thereof may be better understood from the following description of specific embodiments when read in connection with the accompanying drawing, in which:

12 Claims. (01. 333-81) Figure l is a cross-sectional view of a simple embodiment of my invention.

Fig. 2 is a cross-sectional view of a preferred embodiment of my invention.

Fig. 3 is a View taken along line IIII1I of Fig. 2, and

Fig. 4 is a cross-sectional view of still another embodiment of my invention.

As shown in Fig. 1 of the drawing, a hollow tubular guide 5 for electromagnetic waves of a predetermined wave length is provided. The guide 5 is preferably formed of metal or of other suitable material having a high conductive inner surface with the transverse section of the guide 5 being preferabl rectangular with a pair of narrow walls I and 9 and a pair of wide walls, only one wide wall ll being visible in Fig. 1.

An elongated strip I3 is provided which has a conductive surface layer with a resistance substantially greater than that of the inner surface of the guide 5. The strip i3 is positioned within the guide 5 and extends lengthwise thereof substantially perpendicular to the wide walls of the guide 5 and, therefore, parallel to the electric field of the electromagnetic waves conducted through the guide. The strip I3 is preferably formed of an insulating material, such as- Fiberglas Micarta, which is coated on either one or both sides with a conductive material having a resistance substantially greater than that of the inner surface of the guide. A suitable material for this coating is a colloidal suspension of carbon known as aquadag which may be coated on the strip to a thickness of the order of two or three thousandths of an inch to provide a resistance preferably of the order of 200 ohms per square, although resistances of the order of 50 to 500 ohms per square may be satisfactorily employed.

The term ohms per square is commonly used in designating the resistance of a layer, coating or sheet of material to an edgewise current such as would be found in ultra-high frequency work Where currents flow edgewise in a very thin layer at the surface of a conductive material upon which electromagnetic waves are incident. As is well known in this field, the ohms per square resistance of a layer of a material is equal to the resistivity constant of the material divided by the thickness of the layer.

One end of the coated strip 13 engages and is secured to one of the narrow walls I of the guide by a bolt [5. The other end of the strip I3 is engaged by an end of an adjusting screw I! which extends into the guid through an opening in the narrow wall 1, being supported by a cap I9 which covers the opening in the guide and provides a threaded bearing with which the screw is threadedly engaged. Thus, as the screw H is moved inand out, the free end of the strip [3 is moved in an are about the end secured to the wall I of the wave guide 5.

It is apparent that throughout the range of movement of the strip I3, the sides of the strip remain substantially perpendicular to the wide walls of the guide and, therefore, substantially parallel to the electric field of electromagnetic waves conducted through the guide. With the strip parallel to the electricfield, currents flow in the conductive layer on the strip and the resistance of the layer effects an appreciable loss in energy. The amount of energy so absorbed for a layer of a given resistance per square depends upon the area of the layer in the field and the strength of the field at the position of the layer. Now the area of the conductive layer on the strip 13 within the guide remains constant. However, the strength of the electric field of a wave of a fundamental mode customarily used within a rectangular guide is non-uniform across a transverse section of the guide and varies from a minimum adjacent each narrow wall of the guide to a maximum at the center of the guide between the narrow walls. the strip l3 lies fiat against the wall 1 of the guide, the attenuation provided is a minimum. On the other hand, when the free end of the strip 13 is at the center of the guide, the attenation provided is a maximum. The attenuation may then be adjusted between the maximum and minimum values by rotation of the adjusting screw ll.

Preferably, the strip is is, coated with the conductive material only on the side adjacent the wall 1 of the guide. Then when the. strip is fiat against the wall the conductive layer of high resistance is. in the position of the minimum available strength of the field. Consequently, the attenuation provided by the strip is lower than if the strip were coated on the other side or on both sides. If the strip is coated on both sides, however, the maximum attenuation is, increased. I have found that for many cases sufficient maximum attenuation may be obtained with a strip of a practical length when coated on one side only, the width of the strip preferably being but slightly less than the width of the inner surface of the narrow walls. of the guide.

When other practical considerations permit it, the length of the strip l3, from the point of connection to the wall I to the free end, is preferably made of the order of one, wave length of the electromagnetic wave in the. snide or longer. This length is desired to minimize the effect of the strip upon the standing wave ratio and, the effect of, any previously existing standing wave in the guide upon the attenuation provided by the strip.

It has been found that with a strip of sufiicient length, as. of the order of one wave length or longer, the total reflection of energy from the strip in the structure of Fig. 1 is quite small where the original energy is being conducted from left to right in the guide. which is the arrangement for which this structure is most useful. Consequently, the standing wave ratio is changed but very little.

The small amount of reflection app ars to. be due to the cancellation of the energy reflected Consequently when from one point on the strip by the energy reflected from another point on the strip, the spacing of the points being such that the two reflections are opposite in phase, and to the absorption of the energy reflected from points toward the free end of the strip as that reflected energy passes back along the strip. Since the free end of the strip is in. the strongest part of the field, more energy is reflected therefrom than from other portions of the strip, but this reflected energy has to pass back along the length of the strip which absorbs much of it. It is also to be noted that the surfaces at the end of the strip secured to the wall I which tend to produce reflections are in the weakest part of the field so the actual reflections are very small. These may be reduced even further if necessary by tapering the secured end of strip and the portion of the bolt ill in the guide to provide as near a smooth transition from the wall I to the surface of the strip l3 as possible.

In considering the eifect of any standing wave previously existing in the guide, it is to be noted that if the attenuating influence is present only over a distance equal to a small portion of one wave length of the standing wave, the amount of attenuation varies with any movement of the standing wave in the guide. Such variation with movement of the standing wave may be avoided when the attenuator strip is of a length of the order of one wave length or longer.

It is to be furtherunderstood that additional attenuation may be obtained by the use of additional strips secured to either of the narrow walls of the guide, with the strips either spaced from each other lengthwise along the guide or with pairs of strips positioned directly opposite from each other on opposite ones of the narrow walls.

In the preferred embodiment illustrated in Figs. 2 and 3, a pair of flexible strips 2i and 23 are employed which are positioned directly opposite each other within a guide 25. The guide 25- may be similar to the one described in connection with Fig. l with. a rectangular transverse 5 cross-section provided by a, pair of narrow walls 27 and 29 and a pair of wide walls 3i and 33. The strips 2i and 23 are of a material having a conductive layer on either or both sides thereof having a resistance substantially greater than that of the inner surface of the guide and may be formed of the materials described in connection with strip 43 of Fig. 1. Each of the strips 21 and. 23 extends lengthwise of the guide 25 with the sides of the strips substantially perpendicular to the wide walls 3.! and 33 and so, substantially parallel to the electric field of the electromagnetic waves conducted through the guide. The ends of each strip 2} and 23 are slotted so. that they may be slidably secured in engage.- ment with the corresponding narrow wall 2! or 23 of the guide 25 by appropriate bolts. 35. Thewidth of the strips; 21 and 23: are preferably butslightly less than the width of the inner surface; of the narrow walls 21 and 2.9..

An adjustable member 3? extends through the guide '25 parallel to the wide walls 3| and 3:3. thereof. The adjustable member 31 extends; through openings inthe narrow Walls 27' and 29 and corresponding openings incap members 38- and M secured to the. narrow walls, the openings in the cap members providing bearings. in which the adjustable members may be freely rotated. A pair of spacers 43 and 45 are secured tothe adjustable member 3"! at opposite ends thereof by means of set screws 41 and 49- and engage the corresponding cap members 39 and 4| so that lengthwise movement of the adjustable member 31 is prevented. V

The adjustable member 3! is preferably made of an insulating material, such as Micarta, to maintain wave reflections at a minimum and is formed in two parts. The first part consists of an upper stem 5| threaded in one direction with a lower stem 53 of smaller diameter. The second part is a hollow tube 55 which is externally threaded in the opposite direction from the upper stem 5| and fits over the lower stem 53'. The two parts are held together by means of screw 51 and washer 59.

A nut 6|, threadedly engaged with the upper stem 5| of the first part of the adjustable member 31, is secured to the center of the upper flexible strip 2|. A similar nut 63 is threadedly engaged with the tube 55 of the adjustable member 31 and is secured to the center of the lower flexible strip 23. Then with the stem 5| and tube 55 threaded in opposite directions, rotation of the adjustable member 3! causes the centers of the two flexible strips 2| and 23 to be moved simultaneously either toward or away from each other to increase or decrease the amount of attenuation provided. Such an arrangement causes a reflection of but a small amount of energy for waves traveling in either direction and with each of the strips having a length from the adjustable member 31 to each of the ends of that strip of about one wave length or longer, the refiected energy is reduced to an extremely small quantity.

Of course, it is not necessary in all cases to have the amount of attenuation adjustable. Consequently, in some cases it may be desirable to employ an arrangement, such as shown in Fig. 4, wherein two strips 65 and B1 are mounted oppositely within a wave guide 69. These strips are similar to those described in connection with Fig. 2 and each strip is secured to the corresponding narrow wall of the guide 69 at the ends thereof, with the center of each of the strips '65 and 61 being spaced from the corresponding narrow wall by adjustable pins H and 73, respectively. Such an arrangement is very inexpensive and is easily constructed and has the advantages of the structure of Fig. 2 without the adjustability.

Although I have shown the guide as havinga I rectangular transverse section, guides having other transverse sections may be employed. The electric field of the wave conducted by the guide must be non-uniform so the attenuating strip may extend through portions of the field of different strengths with the strip preferably secured at or near a point of minimum field strength.

While I have shown and described certain specific embodiments of my invention, it is obvious that many other variations thereof may be employed without departing from the spirit of the invention. For this reason, I do not intend to limit my invention to the specific embodiments disclosed.

I claim as my invention:

1. In a high frequency system, a hollow tubular guide for an electromagnetic wave, said guide having a conductive inner surface, and an elongated strip having a conductive surface layer on at least one side thereof with a resistance substantially greater than that of said guide surface, said strip being positioned within said guide and extending lengthwise thereof with said side substantially parallel to the electric field of said wave, said strip being secured to a wall of said guide at two points spaced lengthwise of said strip with a portion of the strip intermediate said two points being spaced from said wall.

2. In a high frequency system, a hollow tubular guide for an electromagnetic wave, said guide having a conductive inner surface, an elongated flexible strip having a conductive surface layer on at least one side thereof with a resistance substantially greater than that of said guide surface, said strip being positioned within said guide and extending lengthwise thereof with said side substantially parallel to the electric field of said wave, said strip being slidably secured to a wall of said guide at two points spaced lengthwise of said strip, and means engaging said strip at a point intermediate said two points to effect bowing of said strip inwardly between said two points, said means being adjustable to vary the extent of bowing and thereby vary the amount of attenuation offered by said strip to said wave.

3. In a high frequency system, a hollow tubular guide for an electromagnetic wave, said guide having a conductive inner surface and a rectangular transverse section with a pair of wide walls and a pair of narrow walls, and an elongated strip having a conductive surface layer on at least one side thereof with a resistance substantially greater than that of said guide surface, said strip being positioned within said guide and extending lengthwise thereof with said side substantially perpendicular to said wide walls, said strip being slidably secured to a narrow wall of said guide at two points spaced lengthwise of said strip, and means engaging said strip at a point intermediate said two points to effect bowing of said strip inwardly between said two points, said means being adjustable to vary the extent of bowing and thereby vary the amount of attenuation offered by said strip to said wave.

4. In a high frequency system, a hollow tubular guide for an electromagnetic wave, said guide having a conductive inner surface, a pair of elongated flexible strips, each having a conductive surface layer on at least one side thereof with a resistance substantially greater than that of said guide surface, said strips being positioned within said guide with each strip extending lengthwise of the guide with said side substantially parallel to the electric field of said wave and each strip being slidably secured to a wall of said guide at two points spaced lengthwise of the strip, said strips being mounted directly opposite each other, and means engaging both of said strips intermediate said two points on each to effect bowing of said strips, said means being adjustable to vary simultaneously the extent of bowing of both strips and thereby vary the attenuation offered by the strips to said wave.

5. In a high frequency system, a hollow tubular guide for an electromagnetic wave, said guide having a conductive inner surface and a rectangular transverse section with a pair of wide walls and a pair of narrow walls, a pair of elongated strips, each having a conductive surface layer on at least one ide thereof with a resistance substantially greater than that of said guide surface, said strips being positioned within said guide alongside opposite narrow walls thereof and directly opposite each other, each of said strips extending lengthwise of said guide with said side of the strip substantially perpendicular to said wide walls and being slidably secured to the corresponding narrow wall at two points spaced lengthwise of the strip, and means engaging both accessei of said strips intermediate said two points on each to effect bowing of said strips, said means being adjustable to vary simultaneously the extent of bowing of both strips and thereby vary the amount of attenuation ofiered thereby to said wave.

6. An attenuator for ultra-high frequency waves in a wave guide system of the hollow-pipe type comprising, a. strip of resistive material disposed longitudinally within said guide along the inner surface of a continuous metallic wall of said guide, said strip having its ends supported adjacent said wall and an intermediate portion spaced from said wall and extending toward the center of said guide, said strip between said ends and through said intermediate portion being bent in a gradual curve whereby the impedance of said guide in the direction of propagation of said waves varies gradually to produce a minimum reflection of said waves.

7. An attenuator for high frequency energy in a dielectric wave guide system of the hollow-pipe type comprising a continuous strip of resistance material disposed longitudinally in said guide, said strip having its ends supported adjacent the inner surface of a wall of said guide and it center portion extending toward the center of said guide, and means for selectively adjusting the position or said center portion within said guide.

8. In combination, a waveguide along which a high frequency wave is propagated comprising a hollow rectangular pipe having continuous walls of conductive material, and attenuating means for said wave comprising, a thin strip of resistance material extending longitudinally along the inner surface of one of said walls, said strip having its opposite ends attached to said one wall and its center portion extending a desired distance within said guide, the portions of said strip intermediate said center portion and said ends being in the form of a gradual curve whereby the impedance of said guide varies continuously through said attenuating means in the direction of propagation and reflection of said Wave is reduced to a minimum value.

9. A variable attenuator comprising a section of wave guide of rectangular cross-section in which said guide has unequal cross-sectional dimensions, a pair of resistive strips positioned within said guide substantially opposite each other and substantially perpendicular to the sides of said guide having the longer dimension, and means for moving said strips parallel to the planes of said longer dimensions in opposite directions through substantially equal distances, said means comprising a screw passing through both said strips and means cooperative with said screw for moving one of said strips in one direction and the other in the opposite direction.

16. Apparatus according to claim 9 characterized by the fact that the screw has threads of opposite rotation in the respective regions where it passes through said strips and the cooperative means are nuts secured to opposite sides of said strip, and engaged by said threads of said screws.

11. A variable attenuator comprising a section of wave guide of rectangular cross-section in which said guide has unequal cross-sectional dimensions, a pair of resistive strips positioned within said guide substantially opposite each other and substantially perpendicular to the sides of said guide having the longer dimension, one of said strips being secured at a plurality of points spaced along said strip to an adjacent side of said guide having the shorter dimension, and the other strip being secured at a plurality of points spaced along said last-named strip to the opposite side of said guide of shorter dimension, and means common to said strips for moving said strip parallel to the planes of said -longer climensions in opposite directions through substantially equal distances.

12. A variable attenuator comprising a section of a wave guide, a pair of resistive strips positioned within said guide on both sides of the longitudinal center plane of said guide substantially opposite each other, each of said strips being secured at a plurality of spaced points to the adjacent surface of said guide, and means common to both said strips for bowing said strips towards or away from each other.

WILLIAM E. GOOD.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,594,978 Nelson Apr. 29, 19.52 2,600,466 Bowen June 17, 1952 2,602,852 Hewitt July 8, 1952 

